feat(statusTags): Implement page/breadcrumb tags

Some of our products have been putting ALPHA and BETA tags
into page titles and breadcrumbs by making use of ng-bind-html
in the appropriate name/title fields. This PR formalizes that
as part of the framework.

We've added optional labels to `<rx-page>` via the `status` attribute,
and to breadcrumbs via the `status` attribute.

The framework currently ships with `"alpha"` and `"beta"` tags. If there
is demand for more general ones in the future, we can add those.

This also adds support for per-product custom tags. Products can use
`rxStatusTagsProvider.addStatus` in their `.config()` methods, and then
freely use that status tag in their product.

Closes #638

![screen shot 2014-12-09 at 4 55 07 pm](https://cloud.githubusercontent.com/assets/9376/5366723/340d1d04-7fc4-11e4-8ddd-c706b69cffeb.png)

![screen shot 2014-12-09 at 4 55 16 pm](https://cloud.githubusercontent.com/assets/9376/5366725/3a6b98ce-7fc4-11e4-86d3-4148fb7da371.png)

Should this be `missingKey` (with the K kapitalized?)